Traditionally, bentonite or other clay solids have been utilized to increase the viscosity of the drilling fluid. Today however, there is a growing belief that bentonite or clay suspensions have serious limitations as a drilling fluid base. Of primary importance is the fact that the rheology of bentonite-based fluids is such that the hydraulic horsepower delivered to the bit at a given surface pressure is significantly less than with drilling fluids of lower viscosity and/or solids content. This results in a slower bit penetration rate which in turn increases the drilling costs. In an effort to overcome these problems, dispersants have been utilized to lower apparent viscosities and gel strengths of clay suspensions.
More recently, those working within the industry have attempted to substitute for the clay solids of the older muds various polymeric materials including, for example: cellulose compounds such as carboxyethyl cellulose, carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose, hydroxyalkyl celluloses, alkylhydroxyalkyl celluloses, alkyl celluloses, and alkylcarboxyalkyl celluloses; polyacrylamides; natural galactomannans such as guar gum, locust beam gum, and gums derived from endosperm seeds; and various other polysaccharides.
Additionally, those working with drilling fluids containing the polymeric materials have discovered that it is possible to improve their rheological properties by employing them in combination with different crosslinking agents. Wyant, U.S. Pat. No. 3,079,332, discloses the use of borates as crosslinkers for guar gum and locust bean gum employed as a fluid-loss agent in drilling fluid applications. More recently, Patton et al., U.S. Pat. No. 3,243,000, Chrisp, U.S. Pat. No. 3,301,723, Browning et al., U.S. Pat. No. 3,699,042, and Browning et al., U.S. Pat. No. 3,753,971 have disclosed polysaccharides crosslinked with organic polyelectrolyte ligands and polyvalent metal cations. Certain metal compounds are objectionable from a practical standpoint, however. For example, heavy metals such as chromium are potentially somewhat toxic and may also act as a catalyst poison if residues remain in the formation and become entrained in recovered petroleum products. Other metals such as iron can contribute to more rapid corrosion.
In addition to the drilling fluids employed in drilling subterranean wells, it is known throughout the industry that other fluids can also be utilized for certain specialized applications. For example, fracturing fluids, spacing fluids, plugging fluids, cementing fluids, and completion fluids may be utilized in addition to a drilling fluid to achieve a particular result at one stage or another in the drilling operation. It is recognized by those of ordinary skill in the art, however, that compositions exhibiting properties desirable for some specialized application may not perform satisfactorily where employed for another purpose. Jordan, U.S. Pat. No. 3,483,121, and Free, U.S. Pat. No. 3,974,077, disclose the use of hydroxypropyl guar in fracturing fluids; Kern, U.S. Pat. No. 3,058,909, and Free, U.S. Pat. No. 3,974,077, disclose fracturing fluids comprising guar gum crosslinked with borate ions. Borate ions have proven undesirable for crosslinking polysaccharides utilized in drilling fluids. Unlike fracturing fluids, drilling fluids are recirculated through the well many times. Although much of the larger particulate matter, such as bit cuttings and the like, is removed from the drilling fluid after each cycle through the well, the fine solids content of the fluid becomes progressively greater with continued circulation. Where the drilling fluid comprises polysaccharides crosslinked with borates, it has been found that the borate ions tend to flocculate the fine solids suspended in the fluid. This flocculation or "clumping" characteristic reduces the effectiveness of the drilling fluid, and is considered undesirable. Tiner et al., U.S. Pat. No. 3,888,312, disclose fracturing fluids using organotitanate-crosslinked hydroxyalkyl polysaccharide derivatives.
Therefore, notwithstanding the advances made in drilling fluid technology in recent years, there remains a need for a fluid that will significantly improve bit penetration (and hence the drilling rate), will remain shear stable even after being recirculated through the borehole many times, will reduce the pressure losses due to friction in the drill pipe, will promote better lifting and therefore less grinding of cuttings at the bottom of the borehole, will better lubricate the bit during cutting (thereby increasing bit life), will improve borehole stability, will better lubricate the borehole so as to diminish the chances of hang-ups during "tripping" of the drill pipe, will maintain borehole cuttings in a suspended state during drilling interruptions, and will not flocculate the fine solids suspended in the drilling fluid.